A. Field of the Invention
This invention relates to a method for the preparation of maleic anhydride by the catalytic vapor phase oxidation of saturated hydrocarbons.
B. The Prior Art
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical sythesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs. The prior art discloses a number of processes used in the conversion of organic feed stocks to maleic anhydride.
U.S. Pat. No. 3,293,268 teaches the process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions and in the presence of phosphorus-vanadium-oxygen catalyst. A variety of reactors have been found to be useful and the most commonly used commercial reactor is a multiple tube heat exchanger type reactor. The tubes of such reactors, according to the prior art, vary in diameter from 0.635 centimeters to about 7.62 centimeters and the length varies from about 1 meter to about 3 meters or more. The oxidation is an exothermic reaction and therefore a relatively close control of reaction temperature must be maintained. It is known to be desirable to have the surface of the reactor at a relatively constant temperature and some medium to conduct heat from the reactor is necessary to aid temperature control. The preferred reaction temperature is about 450.degree. C. Temperature control may be effected by the use of mercury, molten lead and the like, but it has been found that eutectic salt baths have been most satisfactory for temperature control. One such salt bath is a sodium nitrate-sodium nitrite-potassium nitrate eutectic mixture. An additional method of temperature control is to use a metal block reactor whereby the metal surrounding the tube acts as a temperature regulating body. The heat exchange medium is kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes have been fabricated of iron, stainless steel, carbon steel, glass and the like. Some reactors contain a preheat zone containing an inert material such as 1/4 inch aluminum oxide pellets, inert ceramic balls, nickel balls or chips and the like.
The fixed tube reactor has not achieved the lower air compression costs per unit of maleic anhydride of a fluid bed reactor due to heat transfer limitations in a conventional fixed tube reactor and a short reaction time in the reactor tubes.
It is an object of this invention to lower air compression costs by increasing the length of the reactor tubes, the concentration of the reactants, and the reaction time.